Cloning and recombinant expression of rat and human kynureninase

Kynureninase [E.C.3.7.1.3.] is one of the enzymes involved in the biosynthesis of NAD cofactors from tryptophan through the kynurenine pathway. By tryptic and CNBr digestion of purified rat liver kynureninase, we obtained about 28% of the amino acid sequence of the enzyme. The rat kynureninase cDNA, isolated by means of reverse-transcribed polymerase chain reaction and hybridization screening, codes for a polypeptide of 464 amino acids. Northern blot analysis revealed the synthesis of a 2.0 kb rat kynureninase mRNA. A cDNA encoding human liver kynureninase was also isolated. The deduced amino acid sequence is 85% identical to that of the rat protein. COS-1 cells were transfected with both cDNAs. The K m values of the rat enzyme, for l-kynurenine and dl-3-hydroxykynurenine, were 440±20 μM and 32±5 μM and of the human enzyme 440±20 μM and 49±6 μM, respectively. Interestingly, COS-1 cells transfected with the cDNA coding for rat kynureninase also display cysteine-conjugate β-lyase activity.